I got to wondering... Could we have built an 8080 microcomputer, like the famous Altair 8800, but in a pocket-sized package? No "wizard behind the curtain" modern chips to make it work. It had to be built with vintage parts and through-hole techniques; just as it would have been back then. It must be user programmable; not simply running a fixed program like the HP-35. And, it's got to have a real front panel, like the Altair with its classic switches and blinkie-lights!
The kits are linked near the end of the page.